The present invention relates to the wire guides of wire type electrical discharge machining devices (hereinafter referred to as "wirecut EDMs") and especially to an improved support structure for the die guides.
FIG. 4 illustrates an example of a wirecut EDM of the prior art. In FIG. 4, numeral 1 indicates a wire electrode fed from a supply bobbin 2; 3, a brake roller directly coupled with an electromagnetic brake 3a for providing a predetermined tension on the wire electrode 1; and 4a, 4b and 4c, idlers for changing the running direction of the wire electrode 1. At 5 is a first top wire guide; 6 indicates a second top wire guide; 7, a top energizer; 8, a top dielectric nozzle; 9, a first bottom wire guide; and 10, a second bottom wire guide. At 11 is a bottom energizer; 12 indicates a bottom dielectric nozzle; 13, a pump for supplying the dielectric; 14, a pulse power supply unit for supplying a pulse voltage to a gap between the wire electrode 1 and the workpiece 15 via energizers 7 and 11; and 16, a wire feed roller.
The wirecut EDM of said structure functions as follows:
First, the wirecut EDM ejects dielectric from the nozzles 8 and 12 in the same axial direction as the wire electrode 1, while simultaneously applying a pulse voltage to a gap between the wire electrode 1 and the workpiece 15 from the pulse power supply unit 14. At the small gap between the wire electrode 1 and the workpiece 15, electrical discharge repeatedly occurs through the medium of the dielectric to melt and cut the workpiece 15 by means of the thermal energy generated at the time of electrical discharge incidental to the vaporization and explosion of the dielectric.
Relative motion between the wire electrode 1 and the workpiece 15 is generally carried out by controlling an X-Y table (not illustrated) numerically. By repeating the electrical discharge and controlling the X-Y cross table as mentioned above, the wirecut EDM of the prior art is designed to machine the workpiece 15 into a given contour.
To apply the said pulse voltage to the wire electrode 1 via the top energizer 7 and the bottom energizer 10, the wire electrode 1 is pressed against the energizers 7 and 10 by the second top wire guide 6 and second bottom wire guide 10 and runs while simultaneously sliding against the energizers 7 and 10.
FIG. 5 illustrates detailed examples of the first top wire guide 5 and the first bottom wire guide 9 in the prior art shown in FIG. 4. The structure shown in FIG. 5 is similar to that disclosed in Kokai Nos. 1981-3148 or 1982-121420, for example. Another known wire guide structure is illustrated in Kokai No. 1988-207524.
Referring to FIG. 5, the numeral 100 indicates a die guide made of an extremely hard material such as diamond or sapphire. Its center is a bearing 100a abutting and supporting the wire electrode 1. The portion gradually increasing in diameter from the center to the upper end is the approach 100b. At 101 is a support member which encloses and centers the die guide. A case 102 accommodates and secures the die guide 100 and the support member 101. As described above, in a wirecut electrical discharge machining process, a pair of wire guides are located on both sides of the workpiece, the workpiece and the pair of top and bottom wire guides are moved relatively to each other by numerical control, and at the same time, pulsed electrical energy and dielectric are supplied to a gap between the wire electrode and the workpiece to generate repeated electrical discharge, erode the workpiece locally, and machine a desired contour with high accuracy. Accordingly, displacement at the support point of the wire electrode must be minimized, and the clearance between the bearing 100a and the wire electrode 1 must be designed to be extremely small, generally between approximately 2 and 10 .mu.m.
In starting wirecut EDMing, the electrode 1 is pulled from the wire supply bobbin 2 in a predetermined wire electrode path including the said wire guides 100. The above process may either be performed manually or automatically using a wire electrode supplier. In either case, for the above reason, it is not easy to insert the wire electrode 1 into the die guide 100 because of the small clearance. In order to smoothly introduce an end of the wire electrode 1 into the bearing portion 100a of the die guide 100, the approach 100b is designed to have a conically smooth curve, and similarly, the inside of the support member 101 is conically shaped to gradually decrease in diameter toward the approach 100b. In addition, to protect the end of the wire electrode 1 from being caught during entry of the wire electrode 1 into the die guide 100, the insides of the die guide 100 and the support member 101 are finished to an extremely smooth surface roughness (e.g., about 1.2.mu.), and the abutment of the die guide 100 and the support member 101 is curved continuously so that no reverse step occurs in the insertion direction of the wire electrode 1. The wire guide of the prior art is generally made in a manufacturing process illustrated in FIG. 6 to form a smooth guide surface without generating a step between the die guide 100 and the support member 101. In FIG. 6, (a) indicates the process of grinding the top and bottom surfaces of a gemstone, such as diamond or sapphire, in parallel; (b) is a process of covering the die guide 100 and performing sintering using a powdered metal (e.g., WC-CO); (c) illustrates the process of drilling the sintered body by laser cutting, etc., (d) shows the process of forming a desired shape by ultrasonic machining, etc.; and (e) is a lapping process (e.g., first the machined surface is lapped with grit of 3 to 6.mu. diameter, then with grit of 0 to 1/4.mu. diameter) for improving the surface roughness and smoothing the cut surface. The process may use metal wires, diamond grit, etc.
The wirecut EDM of the prior art employs a support member 101 formed through metal powder sintering, which is high in machinability as well as mechanical strength, for holding the die guide 100. High mechanical strength is needed to permit the wire guide to sustain the large mechanical forces, such as lateral forces, encountered during machining. However, as wirecut EDMing is performed over a long period of time, the prior art support member 101 will be corroded as shown in FIG. 7. Several factors cause this corrosion. The first factor is an electro-corrosion effect created by the coaction of sludge and water. In wirecut EDMing, electrical discharge occurs in the gap between the workpiece 105 and the wire electrode 1, and part of the workpiece 105 and the wire electrode 1 enter into the dielectric as sludge during dissolution and cooling. This sludge floats in the dielectric during EDMing, but when EDMing stops, it remains in the conical recess formed by the wire guide support member 101 and the die guide 100, and a lot of sludge sinks to the neighborhood of the die guide 100. The dielectric stays in the recess due to its surface tension for a long period of time. For this reason, in the neighborhood of the die guide 100, corrosion is prone to progress by local electrochemical action.
The second contribution to corrosion is attributable to the polarization of the wire electrode 1 and the support member 101, which is assumed to dissolve the positive pole or generate micro-electrical discharges. If the aforementioned corrosion progresses, a reverse step (like a counterbore) will be generated in the insertion direction of the wire electrode 1 as shown in FIG. 7 and prevent the wire electrode 1 from being inserted. Likewise, since the surface roughness of the inside surface of the support member 101 will increase due to erosion, the end of the wire electrode 1 may be caught by the inside surface of the support member 101 when it is inserted. If the corrosion further progresses, the die guide 100 may fall if there is even slight external force. In the above situations, the wire electrode is extremely difficult to insert even manually, and especially when it is inserted by an automatic wire electrode supplier, a serious problem occurs in that the wire electrode cannot be inserted, the subsequent EDM operation cannot be started, and automatic operation comes to a stop. To improve these defects, the prior art shown in FIG. 9 has been suggested (refer to Japanese Kokai No. 1988-193623.)
The art shown in FIG. 9 still has the following problems, although it solves problems of corrosion by using a ceramic material for the support member 101.
To make the device shown in FIG. 9, it is still necessary to use a manufacturing process such as illustrated in FIG. 6. In the process wherein the ceramic material is sintered to cover the die guide with ceramic as shown in FIG. 6(b), the ceramic material must be sintered at 1000.degree. C. to 1500.degree. C.
However, when diamond is used as a die guide material, its surface carbonizes at 200.degree. C., and diamond begins to deteriorate in mechanical strength at 600.degree. C. and higher, and changes in quality as the temperature rises. Accordingly, the die guide will probably change in quality when the ceramic material is sintered.
Furthermore, the processes of drilling, forming and lapping as in FIGS. 6(c), (d) and (e) will be extremely difficult in the case of ceramic.
Therefore, using the ceramic material on the whole support member 101 is not realistic.
Accordingly, it is an object of the present invention to resolve the said defects and provide easily manufactured wire guides for wirecut EDMs.